Directional couplers are used to measure the forward and reflected voltage or power of a transmission line and are well known in the art. These couplers can be used in metering circuits for transmission lines where the forward or reflected voltage or power ratio is desired.
In an ideal situation, directivity is infinite such that only the forward travelling wave is coupled to the forward port and only the reverse travelling wave is connected to the reverse port. Ideal situations are characterized by a perfectly uniform dielectric surrounding the transmission lines.
Typically, directional couplers are of the microstrip or strip line type. These couplers are comprised of conducting strips and a ground plane separated by a dielectric. Due to the nonhomogeneity of the dielectric used in the couplers, the directivity thereof is finite.
Energy propogates through a media (air or dielectric) at a velocity governed by the dielectric constant of the material. The propogation velocity can be formulated as follows: ##EQU1## where, V=velocity of propogation
c=speed of light in a vacuum PA1 Er=dielectric constant relative to air.
Thus a non-momogenous dielectric causes variations in Er which result in variations in the propogation velocity and a finite directivity.
Those in the art have tried to compensate for the finite directivity of the coupler by adding a dielectric cap to a conventional microstrip coupler.
FIG. 1 illustrates a conventional microstrip type coupler. Identification numbers 2 and 3 represent the main and coupled lines. These lines 2 and 3 are copper plated and are disposed on a substrate material 4 of a dielectric material 4 which is mounted on a copper plated ground plane 5. FIG. 2 illustrates a compensated microstrip A type coupler which is essentially the same as that illustrated in FIG. 1, except that a dielectric cap 6 is added to the top of the conductors 2, 3. The dielectric cap 6 was added to compensate for the nonhomogeneity of the dielectric. This technique is described in an article titled, "High Directivity Microstrip Couplers Using Dielectric Overlap" published in IEEE MTTS International Microwave Symposium Digest, 1975, pages 125 to 127.
The technique of using a dielectric cap inherently creates an air gap between the two dielectric substrates which is amenable to warpage and can cause an uneven air gap. The uneven air gap results in a difference in phase velocities, albeit a smaller difference than that of a conventional noncompensated microstrip coupler, nevertheless sufficient to affect the directivity and hence the accuracy of the coupler. Thus, the compensation of the coupler by adding a dielectric cap thereto creates another problem of the uneven air gap between substrates.
Compensation of a stripline type coupler is very similar to the microstrip type. A dielectric cap or overlay is disposed on top of the coupler conductors. One difference with the microstrip is that the cap of the stripline coupler also carries a ground plane. Similar to the compensated microstrip coupler, the stripline coupler is also subject to the problems of the microstrip coupler of uneven air gaps.
Thus, there exists a need to provide a directional coupler which compensates for the finite directivity caused by the non-homgeneity of the dielectric without causing other problems inherent in the dielectric cap couplers. Such a coupler with improved accuracy would be widely received by the industry.